docs: def clearness_index_zenith_independent(clearness_index, airmass,
                                         max_clearness_index=2.0):
      """
      Calculate the zenith angle independent clearness index.
  
      Note:
          [1] Perez, R., P. Ineichen, E. Maxwell, R. Seals and A. Zelenka,
          (1992). "Dynamic Global-to-Direct Irradiance Conversion Models".
          ASHRAE Transactions-Research Series, pp. 354-369
  
      Args:
          clearness_index: numeric
              Ratio of global to extraterrestrial irradiance on a horizontal
              plane
          airmass: numeric
              Airmass
          max_clearness_index: numeric, default 2.0
              Maximum value of the clearness index. The default, 2.0, allows
              for over-irradiance events typically seen in sub-hourly data.
              NREL's SRRL Fortran code used 0.82 for hourly data.
  
      Returns:
          kt_prime : numeric
              Zenith independent clearness index
      """
      if airmass is not None:
          # Perez eqn 1
          kt_prime = clearness_index / (
              1.031 * math.exp(-1.4 / (0.9 + 9.4 / airmass)) + 0.1)
          kt_prime = max(kt_prime, 0)
          kt_prime = min(kt_prime, max_clearness_index)
          return kt_prime
      else:
          return 0